PETROGE APn Y— SANIDIN-TRACHYTE. 5 1 9 



It has a grayisli-green grouiulniaiss quite crystalline in structure, throuj^h 

 which in great abundance are large, black, lustrous biotite crystals and 

 smaller, green columns of hornblende. Besides these there are numerous 

 particles of a white mineral ; some are small and liexagonal in shape and others 

 quite large and crystalline, one in particular having a crust on the surface 

 as if it had filled up a cavity in the rock. In this large mass lies a trans- 

 parent crystal of calcite, which, with many of the smaller masses, effer- 

 vesces in acid. The harder, white particles of more hexagonal form do not 

 effervesce and are j)robably apatite. The rock does not gelatinize in acid, 

 but, treated with nitric acid and amnionic molybdate, gives an abundant 

 yellow precipitate, thus proving the presence of apatite as described in the 

 examination of the thin section. It has 52.02 per cent, of silica, and 23.14 

 per cent, is soluble in hydrochloric acid. The rock is a trachyte, with the 

 accessory and accidental minerals apatite and calcite. 



In the section under the microscope, is seen a feldspathic groundmass, 

 in which are very abundant biotite, hornblende and apatite crystals, with 

 magnetite grains and colorless masses of calcite. The sanidin is in indis- 

 tinct and not very transparent crystals in the groundmass, which seems to 

 consist of the same mineral. The crystals are bright with crossed nicols, 

 when their shape can be made out with tolerable ease, while an occasional 

 good rectangular section may be seen. The hornblende is very abundant 

 in large, pale-green, prismatic crystals, which give very beautiful bright 

 colors, green, red, blue, &c., in polarized light. There are also numerous 

 small crystals, which are quite sharply and distinctly terminated, but they 

 show little dichroism, the majority being cut parallel to the vertical axis. 

 A few sections, however, parallel to the base, have the cleavage lines and 

 are dichroitic. The hornblende has inclusions of magnetite and sometimes 

 of calcite. 



Much more conspicuous is the biotite in its very large and strongly 

 dichroitic, light-brown sections. It is very plentiful in the rock, from long, 

 slender crystals and small fragments to large, brown and almost opaque, 

 hexagonal basal sections. The lines of lamination are distinctly marked, 

 and inclusions of magnetite and calcite are very frequent. Remarkable is 

 a group of hexagonal sections, in which are white, calcite masses, including 



